This invention relates to new variants of electroactive and optoactive polymers formed by the surface chemical modification and derivatization of free-standing and substrate-supported polymer films as well as processes for their formation.
Polyacetylene has been known for some considerable time since its first synthesis by Natta et al. as a black, intractable powder in 1958. See Natta et al., 25 Atti, Acad. Nazl. Linci. Rend, Classe Sci, Fis, Mat. 3 (1958). However, this material has only attracted wide basic and applied research interest since its first reported synthesis as a lustrous, free-standing, polycrystalline film by Shirakawa et al. in the early seventies. See Shirakawa et al., 2 Polym, J. 231 (1971); Shirakawa et al., 4 Polyrn, J. 460 (1973); Ito et al., 12 J. Polym, Sci,; Polym, Chem, Ed. 11 (1974); and Ito et al., 13 J. Polym, Sci,; Polym, Chem, Ed. 1943 (1975). Equally important has been the subsequent discovery by MacDiarmid et al. in 1978 that the material could be made to alter its intrinsic electrical conductivity when exposed to various redox active agents erroneously called "dopants", and that the resulting conductivities could be made to approach that of pure metals. See U.S. Pat. No. 4,222,903 issued to Heeger et al. on Sep. 16, 1980. It has now been demonstrated that polyacetylene can be made to alter its intrinsic electrical conductivity through both chemical redos processes and electrochemical redos reactions. (See Diaz et al., 111 J. Electroanal. Chem. 115 (1980); MacDiarmid et al., 105 Mol. Cryst. Lig, Cryst. 89 (1984)).
Redox processes which lead to charge (electron) transfer from the pristine polymer, i.e. oxidation, give rise to p-type electrical conductivity and redox processes which lead to charge transfer to the pristine polymer, i.e. reduction, give rise to n-type electrical conductivity. In this way, polyacetylene can be made to alter its electrical conductivity from its insulating as-synthesized state, (conductivity of 10.sup.-9 ohm.sup.-1 cm.sup.-1), through a semiconducting state, onto a metallic state (conductivity of 10.sup.3 ohm.sup.-1 cm.sup.-1) through 12 orders of magnitude. This 12 orders of magnitude change in conductivity is achieved for a change in the redos state of 1 to 3 mole percent of available redox active moleties within the polymer.
The availability of polyacetylene in film form and its unusual electrical conductivity has stimulated considerable fundamental and applied science interest in this polymer. Foremost among these are interest in reversible storage batteries (Macinnes, Jr. et al., 3 J. C. S. Chem. Commun. 317 (1981), electronic devices, photoelectrochemical solar cells, and analytical devices (U.S. Pat. No. 4,444,892 issued to Malmros on Apr. 24, 1984).
Unfortunately, polyacetylene suffers from a number of major technological limitations. The pristine material is unstable in ambient temperatures and is very reactive with oxygen, becoming embrittled and undopable. The polymer is also inherently reactive with some of the counter ions which are formed as a consequence of charge transfer doping reactions. This reactivity leads to a precipitous loss of conductivity over time and on the order of days. Additionally, the polymer is intractable and cannot be processed by conventional methods. U.S. Pat. No. 4,499,007 issued to Guiseppi-Elie et al. on Feb. 12, 1985 addresses the issue of stability and provides a method for the stabilization of the polymer in aqueous environments.
Prior art techniques for addressing many of the fundamental limitations of polyacetylene have focused on methods of synthesis of new variants of the polymer. For example, U.S. Pat. No. 4,394,304 issued to Wnek on Jul. 19, 1983 discloses a method for forming a conductive polymer by the in situ polymerization of acetylene within a matrix of a more processable polymer. A similar and related approach is described in U.S. Pat. Nos. 4,510,075, 4,510,076, 4,616,067, 4,705,645 issued to Lee et al., in which acetylene is synthesized in a matrix of a more processable polymer which possesses low unsaturation and is accordingly cross-linkable via Cobalt 60 Gamma-radiation and in various tri-block copolymers. Another approach is that disclosed by Widdegen in U.S. Pat. No. 4,444,970 in which a substituted polyacetylene is formed from the synthesis of regular acetylene monomer in the presence of a substituted acetylene monomer.
The surface of pristine and semiconducting, as well as doped and metallic, free-standing, polyacetylene film has been investigated by Guiseppi-Elie et al., 2 Landmuir 508 (1986). In this work it is demonstrated that the surface of pristine, semiconducting polyacetylene film was hydrophobic with a critical surface tension for wetting of 40.1 mN m.sup.-1 and a dispersion component of surface energy of 58 mN m.sup.-1.
In other related work, Guiseppi-Elie et al., 23 J, Polym, Sci,; Polym, Chem, Ed. 2601 (1985) also demonstrated the surface chemical modification of free-standing polyacetylene film for the introduction of hydrophilic functional groups. In this work the double bonds of the polyacetylene backbone, which are at the near surface, were oxidized using wet chemical oxidative techniques. The result of the surface chemical modification was to alter the energetics of the surface by the introduction of reactive, hydrophilic, surface hydroxyl functional groups. Specifically, Guiseppi-Elie et al. used a method based on permanganate oxidation of surface double-bonds to introduce surface hydroxyl groups to the near surface of preformed polyacetylene film. Using this method, a 30 second treatment in the permanganate solution changed the contact angle made by water at the polyacetylene surface from 72.degree. to 12.degree..
However, in the context of polyacetylene, the consequence of such compositional changes typically is an appreciable sacrifice of electrical conductivity for only modest improvements in stability and processability.
In many technological applications of surfaces there is a need to achieve a topologically uniform, ultra thin organic overlayer of controlled and uniform surface chemistry. Moreover, it is desirable to introduce via adsorption or through specific immobilization, various other molecules which are different in function and purpose to the underlying substrate layer. These overlayer molecules will then confer to the substrate solid the physicochemical properties of the overlayer. Additionally, the overlayer may interact with the substrate underlayer so as to produce some new overall effect, phenomena, or materials property. Such complex, composite, layered structures are called supramacromolecular assemblies.
Of particular importance in such structures are chemical and biological sensors formed from the immobilization of bioactive and catalytic species to the surface of a polymer such as polyacetylene. Polyacetylene is well known to change its electrical conductivity though 12 orders of magnitude upon exposure to, inhibition of, and reaction with, various small redox-active molecules commonly called dopants. Examples of such dopants include ferric chloride, iodine, bromine, and hydrogen peroxide. Polyacetylene used as a sensor in this free-standing film configuration, however, suffers from a major limitation in that its response to environmental redox-active agents is non-specific. That is, any redox active small molecule of appropriate redox potential will induce a change in the polymer. It is extremely desirable to confer reaction specificity and sensitivity of response to polyacetylene films when exposed to these redox active agents.